We just bought a house with an enormous back yard. We love the lawn for dogs and kids running around, picnics and movies in the backyard, and stuff like that. But keeping up with pulling the dandelions, oxalis, and various and sundry other weeds is practically a full-time job.
I'm not really excited about spraying round-up or spreading poisonous pellets all over the place, but I'm starting to wonder if that's my only option.
Our biggest offenders are broadleaf weeds such as dandelion and false dandelions and these big leafy things that clamp down onto the ground and kill the lawn under the leaves, oxalis. (I think. It looks like this: http://www.better-lawn-
We keep it watered, mow regularly, and threw out some grass seed "formulated for partial sun in the NW region" after pulling up so many weeds in one section that it was mostly dirt.
I had the same problem for years. It was always a battle. Then I took some advice from a friend. I aerated and put seed and winter fertilizer down (both Scotts) in September. Then I put fertilizer and pre-emergent down (again Scotts) in the middle of March. Holy crap my yard has never looked so good. I have had no weeds so far this year. My grass is thick and full.
The key was getting the pre-emergent down before the ground reached 55 degrees or so which is when the weeds germinate. It's imperative to get it down at the right temperature.